1. Field of the Invention
The present invention relates generally to semiconductor processing equipment and specifically to systems and methods for improving deposition thickness uniformity and quality.
2. Description of the Related Art
High-temperature ovens, called reactors, are used to create structures of very fine dimensions, such as integrated circuits on semiconductor substrates. One or more substrates, such as silicon wafers, are placed on a substrate support inside the reaction chamber. Both the substrate and support are heated to a desired temperature. In a typical substrate treatment step, reactant gases are passed over the heated substrate, causing the chemical vapor deposition (CVD) of a thin layer on the substrate. Various process conditions, particularly temperature uniformity and reactant gas distribution, must be carefully controlled to ensure a high quality of the resulting layers.
Through a series of deposition, doping, photolithography, and etch steps, the starting substrate and subsequent layers are converted into integrated circuits, with a single substrate producing from tens to thousands or even millions of integrated devices, depending on the size of the substrate and the complexity of the circuits.
Batch processors have traditionally been employed in the semiconductor industry to allow simultaneous processing of multiple substrates, thus economically providing low processing times and costs per substrate. Advances in miniaturization and attendant circuit density, however, have lowered tolerances for imperfections in semiconductor processing. Accordingly, single-substrate processing reactors are now in use, providing improved control of deposition conditions.
In a typical CVD process, one or more reactant gases are passed above the substrate so that they chemically react to deposit a thin layer of material onto the substrate. The reactant materials are ordinarily injected along with a carrier gas, such as hydrogen. The reaction typically produces reaction byproducts that are drawn away by the flow of an inert purge gas, such as hydrogen or nitrogen gas. In epitaxial deposition, the deposited layer maintains the same crystalline structure as the underlying layer or material. Some reactors involve the horizontal flow of reactant gases above the substrate surface. An excellent example of this type of reactor is available commercially under the trade name Epsilon® from ASM America, Inc. of Phoenix, Ariz. Other reactors have so-called showerhead injectors above the substrate, which inject reactant gases downward toward the substrate surface.
One problem with CVD processing is the tendency of some sets of reactants to react in the gas phase before they reach the substrate. This leads to particulate formation, or “gas phase nucleation,” which in turn increases growth rate non-uniformities across the substrate. Not all reactions involve gas phase nucleation. For example, gas phase nucleation is not ordinarily a problem in the epitaxial deposition of silicon (Si), germanium (Ge), and silicon germanium (SiGe). An example of a process in which gas phase nucleation can be a significant problem is the growth of gallium nitride (GaN) for light-emitting diode (LED) applications. Gallium nitride can be grown from the reaction of gallium source gas trimethylgallium (“TMG”) and nitrogen source gas ammonia (NH3) at a temperature of about 1100° C. and a pressure of several hundred Torr.
A prevalent solution to overcome the problem of gas phase nucleation is the use of a showerhead injector (or simply showerhead) that includes separate outlets or holes for the individual reactant gases. This type of injector generally prevents premature mixing until the gases reach the substrate surface, thus reducing gas phase nucleation. However, this approach is very limited because showerhead injectors are process-specific and less useful for conducting a series of different processes and reactions. The optimal size and distribution of the outlets in the showerhead depends upon the nature of the reactant gases used. Also, the effectiveness of this type of injector depends on the process conditions (e.g., process temperature, flow rates of the reactants, etc.). Deviations from the optimal size and distribution of the injector outlets and optimal process conditions can result in loss of uniformity control of the processed wafer.
Another problem with using a showerhead is that it is often incompatible with high temperature CVD processes (e.g., above 800-900° C.). At elevated temperatures (e.g., above 600° C.), the substrate and substrate holder radiate a significant amount of heat toward the showerhead. The resultant temperature increase of the showerhead can cause reactant gases to decompose prematurely in the injector holes. This leads to clogging of the injector holes and loss of control over the growth rate of the deposited layers on the substrate. Water-cooling of the showerhead can alleviate this problem to some extent, but it further complicates the showerhead construction and typically does not sufficiently suppress this problem.